When Japan identified its first local case of dengue in 70 years last month, it became the latest in a string of countries to battle the tropical fever. The most likely importers of the disease: international travelers.

A type of mosquito called aedes albopictus probably got the virus through people arriving from overseas, and infected others, according to Japan’s health ministry. The number of reported cases in the country surpassed 100 last week.

The outbreak in Japan is an extension of the dengue pandemic that has moved around the world over the past 30 years to Europe, the U.S. and China, said Duane Gubler, professor of the emerging infectious diseases program at Duke University- National University of Singapore Graduate Medical School. Urbanization in developing nations and increased global travel have helped fuel the spread.

“Tens of millions of people pass through airports a year,” Gubler said. “That’s why we see dengue moving around.”

There now are more than 50 to 100 million cases of dengue worldwide each year, and 3 billion people live in dengue endemic countries, according to the World Health Organization. Dengue, transmitted by the bite of an infected mosquito, can’t be spread directly from person to person. It causes flu-like symptoms, joint pain and high fevers.

Early Recognition

A potentially lethal complication called severe dengue — or dengue hemorrhagic fever — can also cause abdominal pain, persistent vomiting, bleeding and breathing difficulty. With proper medical care and early recognition, case-fatality rates are below 1 percent, the WHO said.

Incidents of dengue have grown “dramatically” in recent years, and more than 40 percent of the global population is at risk, according to the WHO. The disease is now endemic in more than 100 countries, compared with only nine countries that had experienced severe dengue epidemics before 1970, it said. Countries like India, with warm climates, have been especially susceptible.

The disease appeared in Florida in 2009 for the first time since 1934, and cases among travelers returning to the U.K. tripled last year from a year earlier. In 2010, the local transmission of dengue was reported for the first time in France and Croatia.

The disease occurred in China’s Western province of Yunnan last year, according to the WHO. There are no vaccines or specific medications and patients are advised to seek medical care, while getting rest and fluids. Takeda Pharmaceutical Co. of Osaka, Japan, Sanofi and Merck & Co. are among companies trying to develop a dengue vaccine.

The Japanese outbreak will probably be small and sporadic because the aedes albopictus mosquito lives outdoors and isn’t an efficient vector, or transmitter, said Gubler. Countries with tropical climates must cope with another dengue-carrying mosquito, aedes aegypti, which has adapted to human environments and only feeds on people, he said.

Also, mosquitoes carrying the virus may not make it through the coming winter, Masanori Kameoka, a virologist at Kobe University said earlier this month.

Increased Tourism

Dengue fever may have been around in Japan before the latest outbreak, and gone undiagnosed because physicians are unlikely to suspect it and there are few laboratories for its detection, said Yoshihiro Takayama, an infectious disease expert who helped Japan’s government develop a medical system to cope with the 2009 flu pandemic. Annual cases may have been over a thousand, instead of the 200 cases Japan reports each year in people who have caught it overseas, he said.

Yoyogi Park in central Tokyo is thought to be the primary source of most of the Japanese infections. Japan had an estimated 1.3 million tourists in July alone, a 27 percent increase from a year earlier, according to the Japan National Tourism Organization. More than a million people have visited Japan monthly from March to July.

Dengue is widespread in Puerto Rico and many popular tourist destinations in Latin America, Southeast Asia and the Pacific islands, according to the Centers for Disease Control and Prevention.

“The interaction between Japan and Southeast Asia is becoming more active, which means that the outbreak was imminent,” said Takayama. “I’m impressed doctors suspected dengue fever and decided to test patients for it.”

–With assistance from Rin Ichino in Tokyo.

To contact the reporter on this story: Kanoko Matsuyama in Tokyo at kmatsuyama2@bloomberg.net. To contact the editors responsible for this story: Anjali Cordeiro at acordeiro2@bloomberg.net. 

Tags: health, japan
Photo Credit: Passengers arriving at Tokyo's Narita Airport. David McKelvey / Flickr